The principal technique employed in ultrasonic distance measurement in an air or water operating medium is to transmit a burst of ultrasonic energy into the operating medium and measure the elapsed time between the time of transmission and the time of receipt of an echo from a distant target. With the knowledge of the speed of sound in the operating medium and the time taken to travel from the transmitter to the target and back to the receiver, the distance to the target can be calculated.
A typical measuring system of this type consists of one or more electroacoustic transmitting transducers and one closely located receiving transducer. The transducers are commonly piezoelectric devices which are pulse driven and tuned to exhibit a sharp resonant frequency within the range of 20 KHz to 400 KHz in air and up to several MHz in liquids. The choice of frequency depends on the required maximum range, accuracy and finest resolution.
The simplest configuration which is commonly employed consists of a single piezoelectric transducer performing the dual function of transmitter and receiver. However, this arrangement suffers from a major disadvantage in that short range measurements are impossible without complex (and often unreliable) signal processing. The reason for this is that a short electrical transmitter driving pulse causes a much longer ringing burst of ultrasonic energy, thereby limiting the receipt of the echo signal until the ringing decays.
A means of overcoming the above described disadvantage is to separate the transmitter and receiver so that they are completely acoustically isolated from one another. This approach is most successful when there is wide spacing between transmitter and receiver, but such wide spacing introduces other disadvantages inherent in having large dimension apparatus.
In many applications requiring compactness, there is a necessary requirement for the transmitting and receiving transducers to be closely spaced and to share the same substrate medium. Sufficient isolation between such closely spaced transmitter and receiver is difficult to achieve even when sharing a substrate medium of acoustic insulating material. Direct transmission through the operating medium, surface waves across the substrate medium, and transmission of internal spurious ultrasonic signals through the substrate medium also make it impossible to provide the desired acoustic isolation between transmitter and receiver. This interference between transmitter and receiver is commonly termed “cross-talk”.
Cross-talk becomes a serious problem in the measurement of short distances when the echo signal is received before the natural oscillation of the pulse driven transmitter has time to decay and transmits through the substrate medium to distort or swamp the received echo signal. The result is to limit the capability of short distance measurement. This effect is called “dead band”. In performance specifications of this type of distance measuring device, a small dead band is extremely important.
Furthermore, the choice of waterproof piezoelectric transducers to withstand a harsh environment, say, in sewer pipes has limited the prior art systems to use of particular waterproof cylindrically shaped devices with a piezoelectric transducing diaphragm on one end and a sealed back face. The inertia of a vibrating diaphragm of the transmitting transducer is sufficient to cause spurious ultrasonic signals from the side and back faces. Conversely, the side and back faces of the receiving transducer become sensitive to the spurious ultrasonic signals through the substrate medium. This increases the cross-talk effect.
Encapsulation of the transmitting and receiving transducers in an acoustically attenuating substrate medium provides a significant reduction in cross-talk. However, sufficient isolation between transmitter and receiver is still difficult to achieve in compact configurations.
The problem of cross-talk is further complicated in systems employing a plurality of transmitters in an array surrounding a single receiver. Whilst the multiplicity of transmitters provides the advantage of cooperating to form a narrower beam angle, and therefore improved directivity, the problem of cross-talk remains.
It is an objective of the invention to provide an arrayed system encapsulated in an acoustically attenuating substrate medium that can be geometrically configured and driven with phased transmission pulses to substantially eliminate cross-talk and so substantially reduce dead band.